Conventionally, there is technology that realizes synchronous transmission by merely performing transmission jitter control with the use of a buffer included in a reception device, under the condition that the input bit rate at which packets are input to a transmission device and the transmission bit rate of packet on a transmission line are both a constant bit rate (CBR) and both the same value (see patent document 1).
In the embodiments of the present invention, “constant bit rate” means that the average bit rate per fixed period is a constant value, and “variable bit rate” means that the average bit rate per certain fixed period varies.
The following describes an outline of the technology disclosed in patent document 1 for realizing conventional synchronous transmission with reference to FIG. 17. FIG. 17 shows the structure of a reception device that realizes such conventional synchronous transmission.
A reception device 1000 includes an input terminal 1001 that receives an input of a packet from a transmission line, a conditioning circuit 1002 with a transmission jitter buffer (not depicted) provided therein, a system decoder 1003, a timestamp acquisition circuit 1004, a PLL (Phase Locked Loop) circuit 1005, an output terminal 1006 for externally outputting packets, and an output terminal 1007 for externally outputting a system clock.
In the following, BuffRx(t) is a stored amount function that expresses the amount of packets that are stored in the transmission jitter buffer in the conditioning circuit 1002 at time t. G(t) is a rate function that expresses the input bit rate at which a packet is input to the transmission jitter buffer at time t, and F(t) is a rate function that expresses the output bit rate at which a packet is output from the transmission jitter buffer at time t.
The rate function G(t) is 0 until time T0, and c (c being a constant value) at or after time T0, and the rate function F(t) is 0 until time T1, and c (c being a constant value) at or after time T1 (FIG. 18A).
The stored amount function BuffRx(t) at time t, which is at or after time T1, is expressed by the following expression (1)
                              Expression          ⁢                                          ⁢                      (            1            )                          ⁢                                                                                                BuffRx          ⁡                      (            t            )                          =                                                            ∫                0                t                            ⁢                                                G                  ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                      -                                          ∫                0                t                            ⁢                                                F                  ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                              =                                                                      ∫                                      T                    ⁢                                                                                  ⁢                    0                                                        T                    ⁢                                                                                  ⁢                    1                                                  ⁢                                                      G                    ⁡                                          (                      t                      )                                                        ⁢                                      ⅆ                    t                                                              +                                                ∫                                      T                    ⁢                                                                                  ⁢                    1                                    t                                ⁢                                                      G                    ⁡                                          (                      t                      )                                                        ⁢                                      ⅆ                    t                                                              -                                                ∫                                      T                    ⁢                                                                                  ⁢                    1                                    t                                ⁢                                                      F                    ⁡                                          (                      t                      )                                                        ⁢                                      ⅆ                    t                                                                        =                                                                                ∫                                          T                      ⁢                                                                                          ⁢                      0                                                              T                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      c                    ⁢                                          ⅆ                      t                                                                      +                                                      ∫                                          T                      ⁢                                                                                          ⁢                      1                                        t                                    ⁢                                      c                    ⁢                                          ⅆ                      t                                                                      -                                                      ∫                                          T                      ⁢                                                                                          ⁢                      1                                        t                                    ⁢                                      c                    ⁢                                          ⅆ                      t                                                                                  =                                                                    ∫                                          T                      ⁢                                                                                          ⁢                      0                                                              T                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      c                    ⁢                                          ⅆ                      t                                                                      =                                  constant                  ⁢                                                                          ⁢                  value                                                                                        (        1        )            
In expression (1), since the data amount of packets input to the buffer from time T1 to time t is the same as the data amount of packets output from the buffer during the same period, expression (1) shows that the stored amount function BuffRx(t) is constant from time T1 onward (FIG. 18B). In other words, the stored amount of packets in the buffer is constant.
In consideration of this, the conventional technology realizes synchronous transmission by controlling the speed at which packets are output from the buffer so that the stored amount of packets in the buffer is constant.    Patent document 1: Japanese Patent Application Publication No. H8-139704